If the Predictor continues to work in this circumstance, it would be evidence against MWI, since on MWI there are two futures -- one in which you push the button and one in which you don't -- that both presumably send signals back to the Predictor. Since only one of these signals can determine the Predictor's behavior, it will get the prediction wrong for some branches. Consistently finding that you are not in one of these branches becomes more and more improbable as the number of trials increases.